The electrophotography process used in some imaging devices, such as laser printers and copiers, utilizes electrical potentials between components to control the transfer and placement of toner. These electrical potentials create attractive and repulsive forces that tend to promote the transfer of charged toner to desired areas while ideally preventing transfer of the toner to unwanted areas. For instance, during the process of developing a latent image on a photoconductive surface, charged toner particles may be deposited from a biased developer roller onto latent image features (e.g., corresponding to text or graphics) on the photoconductive surface having a surface potential that is lower in magnitude than the developer roller. At the same time, the charged toner particles may be prevented from transferring or migrating to more highly charged areas (e.g., corresponding to the document background) of the same photoconductive surface. In this manner, imaging devices implementing this process may simultaneously generate images with fine detail while maintaining clean backgrounds.
The precise magnitudes of these electrical potentials vary among devices and manufacturers. In general, however, a laser or imaging source is used to illuminate and selectively discharge portions of a photoconductive surface to create a latent image having a lower surface potential than the remaining, undischarged areas of the photoconductive surface. The developer roller is biased to some intermediate level between the discharge potential of the latent image and the surface potential of the undischarged photoconductive surface. The toner may be charged triboelectrically and/or via biased toner delivery control components, such as a toner adder roll, a doctor blade, and a developer roller. The developer roller supplies toner to develop the latent images on the photoconductive surface. The developed image is ultimately transferred onto a media sheet, typically by employing yet another surface potential that attracts the toner off of the photoconductive surface (or an intermediate transfer surface) and onto the media sheet where it is ultimately fused.
The various surface potentials may be optimized to strike a balance between maintaining clear backgrounds while producing quality images with fine detail. For example, the surface potential of a developer roller may be optimized to develop images with a desired toner density. Another variable termed a “white vector” may be optimized as well. White vector refers to the difference between the surface potential of the developer roller and the surface potential of undischarged portions of a photoconductive surface. An optimal white vector achieves certain desirable characteristics, one of which is to provide a clean media sheet with little or no appreciable background toner in areas other than where printing is desired. Very large white vector values may adversely affect the density of deposited toner and detail of a resulting image. Conversely, as white vector values fall, unwanted background may begin to appear.
Even when these various surface potentials are optimized, image quality may be improved by further optimization of imaging power. Imaging power affects the formation of the latent image on a photoconductive surface. Consequently, incorrect imaging power settings may adversely affect image quality and halftone linearity. In some cases, the discharged latent image may not attract enough toner while in other cases, too much toner is attracted. The effects that are produced by changes in imaging power may vary depending on the surface potentials used in the image formation process. Thus, the imaging power may need to be optimized while taking into consideration the optimization of the various surface potentials. By the same token, optimization of the imaging power may affect the optimization of the various surface potentials. As a result, improved image production may dictate that these various operating points be optimized in consideration of one another.